This invention relates to amorphous silica and most especially to silica gels useful in catalysis. The invention also relates to a method for preparing such silica gels, and to catalytic processes wherein the gels are utilized as catalysts or catalyst components.
Silica gel is but one of several forms of amorphous silica and, like the other forms, is an essentially anhydrous polymer of silica. Silica gel may be prepared in a number of ways, one of which is to form an aqueous silicate solution, usually an aqueous sodium silicate solution, followed by acidifying the solution to form a silica hydrogel, which, upon drying, yields a silica gel (or silica xerogel) product. Alternatively, one may form the gel by removal of the cation associated with the silicate anion by ion exchange, followed by drying of the resultant hydrogel.
The formation of the hydrogel, by ion exchanging, acidifying, or otherwise destabilizing an aqueous silicate solution, is caused by a polymerization reaction that may be visualized as the formation of a silica network which envelops water like a sponge. Upon heating, the water is removed, leaving microscopic cavities or pores in the locations formerly occupied by water. The resulting product, termed either a silica gel or silica xerogel, is highly useful, as for example as an adsorbent in gas masks or as a desiccant.
As is the case with many crystalline siliceous materials, such as mordenite or Zeolite Y described in U.S. Pat. No. 3,130,007, silica gel is known as a catalyst component. However, unlike these crystalline materials, silica gel is not itself catalytically active. Instead, silica gel is most frequently used in catalysts as a relatively inert support material upon which any of a number of catalytically active metals are deposited.
Despite its usual inertness, some attempts have been made to produce reactive silicas. Among these are the achievements of M. J. D. Low and H. Mark, who have authored several articles directed to silicas of improved chemisorption properties produced by treating conventional silicas with various chemical agents (for instance, trimethoxymethane and certain dienes). The articles of Low and Mark relating to reactive silicas include "Reactive Silica VIII. Methoxylation of Silica Using Trimethoxymethane" set forth in the Journal of Catalysis, vol. 44, pp. 300 to 305 (1976), "Reactive Silica X. Ethylene Sorption and Polymerization" set forth in the Journal of Catalysis, Vol. 48, pp. 104 to 110 (1977), and "Reactive Silica XII. The Sorption and Polymerization of Several Alkenes" set forth in the Journal of Catalysis, Vol. 50, pp. 373 to 378 (1977). M. J. D. Low has also authored a paper relative to reactive silicas in conjunction with A. G. Severdia entitled "Reactive Silica XIII. Activation of Silica by Pyrolizing Chemisorbed HSiCl.sub.3 " as set forth in the Journal of Catalysis, Vol. 54, pp. 219 to 222 (1978).
The efforts of the art with respect to silica gels have not resulted in a commercially attractive silica gel having highly catalytic properties. From the viewpoint of commercial preparation, it would be especially desirable if not only a silica gel of highly active catalytic properties were available but also a method of preparing such a silica gel without resorting to either expensive and/or corrosive chemicals or preparations involving conversion of inactive silica gels to a catalytically active form. More specifically, it would be desirable if a direct preparation method were available for producing a catalytically active silica gel without the necessity for the intermediate formation of an inactive silica gel.
Accordingly, it is a major object of the present invention to prepare a novel amorphous silica of high catalytic activity, particularly in the form of silica gel, and to do so directly, without the necessity for converting an inactive silica gel to an active form. It is a further object of the invention to provide chemical processes wherein the silica gel of the invention is utilized to catalytically promote one or more chemical reactions. It is yet another object of the invention to prepare the silica gel of the invention with relatively inexpensive and noncorrosive chemicals. It is yet another object to provide a novel silica hydrogel, which, upon heating, or upon washing followed by heating, yields the catalytically active silica gel of the invention. These and other objects will become more apparent to those skilled in the art in light of the following description of the invention.